Riccardo Giussani

Partial discharge pulse propagation, localisation and measurements in medium voltage power cables

This paper considers the measurement and propagation of partial discharge pulses on distribution class power cable circuits, with the idea of attempting to determine discharge location sites on cables based on the parameters of individual pulses. Single-ended discharge location techniques based on partial discharge pulse parameters and shape will not be as accurate as time-domain reflectometry methods but can be done on-line without the need for double-ended techniques. Power cables used for the transmission of 50/60 Hertz electrical power, are by design not intended to carry transients or partial discharge pulses. The geometry and construction of such power cables present a transmission line that can heavily attenuate and distort the partial discharge pulses, making their detection and discrimination all the more difficult. Experimental and field work has been carried out to develop basic knowledge rules to describe how the time-domain parameters of individual pulses alter as a function of the distance propagated from the discharge sites for medium voltage power cables.

Comparison of IEC 60270 and RF partial discharge detection in an electromagnetic noise-free environment at differing pressures

Measuring partial discharge according to IEC60270 is a well established and widely accepted method. Unfortunately it presents some limitations especially when it is necessary to monitor the system under load conditions and when the localization of the discharge sources is demanded. To overcome these problems, other techniques have been developed in the last few years and one of the most promising methods is the on-line detection and localization of partial discharge using RF techniques. Work in this area, especially in the UHF band is an area that has been developing significantly in recent year. This paper describes initial results from the measurement system recently installed at the University of Manchester that is being used to compare IEC60270 and RF UHF detection methods in a controlled electromagnetic noise-free environment at a variety of pressures (10-101.3 kPa). The use of varying pressure reflects the need for partial discharge testing of equipment that will be operated in an aerospace environment as more-electric aircraft see high voltage power systems being deployed. The system is housed in a screened room built according to EN 50147-1 that provides a screening factor of at least 100 dB in the detected frequency band (1MHz-3GHz). The test set is equipped with a LEMKE LDS-6 PD measuring system, an ADVANTEST Spectrum Analyzer and an Agilent 6GHz oscilloscope. Several inhouse and commercial sensors have been tested with a range of scanning antennas. The facility has been developed to provide an understanding of RF detection in both standard and low pressure environments and to provide a tool to study the RF spectra emission associated with different PD sources. It is hoped that this will lead to the design of a RF-based PD detection monitoring system for power electronic systems used in a range of applications.

Optimization of a High-Frequency Current Transformer Sensor for Partial Discharge Detection Using Finite-Element Analysis

High-frequency current transformer (HFCT) sensors are widely used for partial discharge detection due to their versatility, high sensitivity, and wide bandwidth. This paper reports on a finite-element analysis (FEA) methodology that can be employed to optimize HFCT performance. The FEA model consists of accurate 3D representations of the sensor components. Two different FEA software modules were used in order to cover the wide operating frequency range of the sensor. The simulation computes the frequency response of the sensor in the range 0.3-50 MHz for various HFCT geometric and material parameters, specifically the number of winding turns, spacer thickness, aperture size, and core material. A prototype HFCT was constructed and the measured response compared with that of the simulation. The shapes of the responses were similar, with the simulated sensitivity being higher than the measured sensitivity by 1 dB on average. The measured low-frequency cutoff of the sensor was found to be only 0.05 MHz lower than that of the simulation.

Detection of corona with RF methods and spectra analysis

High voltage power electronic systems are increasingly being used in applications such as renewable energy generation and in transportation systems. Such high voltage systems are often placed in compact packages to minimize weight and volume. In minimizing package size, there are consequences on the design of the high voltage insulation system which is likely to be operated closer to design limits. This can lead to an increasing number of failures due to tracking, partial discharges or disruptive discharge. An opportunity therefore exists to develop a monitoring system that can detect a range of electrical discharges and which is able to work in environments polluted by electromagnetic noise generated by rapid switching of power electronics under any load conditions. Of the methods that have been developed in recent years (optical, ultrasonic, RF etc.) the ones based on RF techniques are the most promising. To enable an easy differentiation between the RF signals produced by a PD and the random environmental noise it is crucial to understand the type of emissions that can be generated by different kind of discharges. In this experimental work carried out in a controlled electromagnetic noise-free environment, arbitrary corona discharges (of known magnitude and polarity) have been detected with RF techniques and the signals have been analyzed. Results relative to positive and negative corona generated with blade-plane gap specimen at varying voltages (1-4kV) and at differing pressures (11-101.3 kPa) are reported. This is the first part of a study targeted to classify the spectral emissions associated with different PDs; future work will analyze emissions due to voids, floating particles, tracking etc. The final aim of this project is the development of a system capable of a monitoring a range of discharge types in high voltage power converters including those operated at altitude in aerospace applications.

On-line partial discharge condition monitoring of complete networks for the pulp and paper industry: Challenges and solutions explained through case studies

Outages within the paper industry lead to significant costs to operators. To reduce unplanned outage and losses condition monitoring (CM) techniques can be implemented to determine asset health and help target maintenance where it is most required. The challenges faced by the paper industry to implement condition based maintenance (CBM) on their medium voltage (MV) and high voltage (HV) networks and solutions for on-line partial discharge (OLPD) monitoring are due to the range and number of assets present and geographical spread coupled with problems of retrofitting the technology on pre-existing plants. The implementation of OLPD testing and/or monitoring can vary based on several factors: network topology, connected assets, switchboard type, owner budget and owners desired local engineer involvement. This paper discusses different approaches to implementing an OLPD test program, ranging from simple handheld survey units to complete network monitoring. This paper focuses on case studies of the deployment of OLPD testing and monitoring solutions at several paper sites within the U.S. The case studies will present diagnostic testing of unshielded MV cable at one paper mill up to full permanent OLPD monitoring systems at two others.